1. Field of the Invention
The present invention relates to a semiconductor device that protects a circuit element from an overvoltage.
2. Description of the Related Art
In a conventional semiconductor device, an N-type epitaxial layer is formed on an N-type semiconductor substrate. A P-type diffusion layer is overlapped onto an N-type diffusion layer formed on the epitaxial layer. Then, an anode electrode is formed on the P-type diffusion layer, and a cathode electrode is formed on a back surface of the substrate to form a Zener diode using a PN junction of both diffusion layers. A P-type guard region is formed around the P-type diffusion layer and another guard region is formed on its outer side. A Schottky barrier metal layer is formed in such a way to come in contact with the epitaxial layer surrounded by both guard regions. Then, a Schottky barrier diode is formed of a silicide of the Schottky barrier metal layer and the epitaxial layer. In the conventional semiconductor device, the Zener diode and the Schottky barrier diode are connected in parallel to achieve a reduction in a forward voltage (Vf) of a device itself. This technology is described for instance in Japanese Patent Application Publication No. Hei 8 (1996)-107222 (Pages 2 to 4, FIG. 1).
In a conventional semiconductor device, P-type diffusion layers with a high impurity concentration are formed on a surface of an N-type semiconductor region and a P-type diffusion layer with a low impurity concentration is formed between the diffusion layers with a high impurity concentration. An electrode formed on the surface of the N-type semiconductor region makes an ohmic contact with the P-type diffusion layer with a high impurity concentration to form a Schottky barrier between the P-type diffusion layer with a high impurity concentration and the P-type diffusion layer with a low impurity concentration. In a forming region where the P-type diffusion layer with a high impurity concentration is formed, a Zener diode is formed using a PN junction. On the other hand, in a forming region where the P-type diffusion layer with a low impurity concentration is formed, a diode, which includes the Zener diode and a Schottky barrier, is formed. This structure decreases free carriers (holes) to be injected into the N-type semiconductor region from the P-type diffusion layer and reduces free carriers (holes) to be stored in the vicinity of the PN junction region. This results in a reduced reverse recovery current density. This technology is described for instance in Japanese Patent Application Publication No. Hei 9 (1997)-121062 (Pages 5-6, FIG. 2).
In a conventional planar type semiconductor device, an anode electrode is formed on a top surface of a P-type semiconductor region formed in an N-type semiconductor region. A conductive field plate, which is connected to the anode electrode, is formed on a top surface of the N-type semiconductor region. Moreover, an equipotential ring electrode and the conductive field plate formed on a top surface of the N-type semiconductor region are connected to each other by a resistive field plate. Then, a thickness of an insulation film which is positioned at a lower portion of a boundary between the conductive field plate and the resistive field plate is thickened, and a thickness of an insulation film, which is positioned at a lower portion of the resistive field plate on the equipotential ring electrode, is thinned. This structure intensifies an effect of the resistive field plate to reduce a curvature of a depletion layer placed at the lower portion of the boundary between the conductive field plate and the resistive field plate. This achieves an improvement in a breakdown voltage characteristic in a region where an electric field concentration is apt to occur. This technology is described for instance in Japanese Patent Application Publication No. Hei 8 (1996)-130317 (Pages 3 to 6, FIGS. 2 and 4).
As mentioned above, according to the conventional semiconductor device, the Zener diode and the Schottky barrier diode are connected in parallel in one device. This structure makes it possible to achieve a low voltage drive by use of a forward voltage (Vf) characteristic of the Schottky barrier diode. However, in the Schottky barrier diode, an epitaxial layer is used as a channel for a main current. This causes a problem in which a parasitic resistance is high in the epitaxial layer, thereby making it impossible to reduce an ON-resistance value.
Moreover, according to the conventional semiconductor device, in the Zener diode, the P-type guard region is formed at a lower portion of an end portion of an anode electrode formed on a top surface of the epitaxial layer. Likewise, in the Schottky barrier diode, the P-type guard region is formed at a lower portion of an end portion of the Schottky barrier metal layer. This structure protects a region where the electric field concentration is apt to occur using the P-type guard region. However, in a structure in which the P-type guard region is formed on an outermost periphery, when a reverse bias is applied, the curvature of the depletion layer is likely to be changed in the vicinity of the end portion of the anode electrode and that of the Schottky barrier metal layer. Particularly, when the above end portion is placed in the vicinity of a termination region of the depletion layer, the change in the curvature of the depletion layer is increased. As a result, there is a problem in which the electric field concentration is apt to occur in the region where the curvature of the depletion layer is changed, thereby making it difficult to achieve a desired breakdown voltage characteristic.
Furthermore, according to the conventional semiconductor device, at a Zener diode operating time, free carriers (holes) which are minority carriers are excessively stored in the N-type epitaxial layer. Then, when the Zener diode is turned off, it is necessary to remove the stored free carriers (holes) from the P-type diffusion layer. At this time, a concentration of the free carriers (holes) in the vicinity of the P-type diffusion layer is high, thereby leading to an increase in an absolute value of a rate of a change of a reverse recovery current in time (di/dt). Accordingly, there is a problem in which a destruction of a protection diode is caused by the rate of change of the reverse recovery current in time (di/dt).
Furthermore, a low forward voltage (Vf) characteristic of the Schottky barrier diode is used to allow the protection diode to operate before the circuit element operates when the overvoltage is applied to the circuit element, thereby preventing the circuit element from being destroyed. In such a case, there is a problem that the forward voltage (Vf) characteristic of the Schottky barrier diode is made too low, for example due to the influence of the Schottky barrier metal layer formed on the surface of the epitaxial layer or due to other reasons, thereby causing an increase in a reverse off-leak current.
Furthermore, the low forward voltage (Vf) characteristic of the Schottky barrier diode is used to allow the protection diode to operate before the circuit element operates when the overvoltage is applied to the circuit element, thereby preventing the circuit element from being destroyed. In such a case, there is a problem that the forward voltage (Vf) characteristic of the Schottky barrier diode is made too low, for example due to the influence of the configuration or the like of the Schottky barrier metal layer formed on the surface of the epitaxial layer, and the like, thereby causing an increase in the reverse off-leak current.